1. Field of the Invention
The present application relates to battery packs formed of electrically interconnected cells, and in particular, to lithium (e.g., lithium-ion, lithium-polymer, etc.) cells arranged into modules with elastic bladder elements interposed with the cells.
2. Description of the Related Art
Battery packs provide power for various technologies ranging from portable electronics to renewable power systems and environmentally friendly vehicles. For example, hybrid electric vehicles (HEV) use a battery pack and an electric motor in conjunction with a combustion engine to increase fuel efficiency. Battery packs are formed of a plurality of electrochemical cells. Although nickel metal hydride (Ni-MH) cells are commonly used to form battery packs for HEV applications, lithium-ion (Li-ion) cells are increasingly used in HEV applications since they provide roughly twice the power and energy density of a Ni-MH cell.
Lithium-ion cells are sometimes provided in a cell housing having a cylindrical or prismatic (rectangular) shape. Alternatively, such cells may be in the form of a so-called pouch cell. Regardless of shape, the cell may include electrodes (for example, a cathode, an anode and an intermediate separator provided in a stacked arrangement) that are rolled in the form of a so-called jelly roll and are placed in the cell housing along with an electrolyte.
To construct a power-producing electrical system, multiple cells are arranged in stacks and are connected electrically in series or in parallel. The voltage of the cell is dependent on the cell chemistry, the current is dependent on the rate of ion transfer between the cathode and anode, and the capacity depends on the total surface area of the cell. To maintain cell capacity over the life of the cell, it is important to maintain a uniform distribution of pressure across a surface of the cell.
However, some cell configurations are subject to cyclical changes in volume as a consequence of variations in the state of charge of the cell. For example, in some instances, the total cell volume may vary as much five to six percent or more during charge and discharge cycling. Thus a need exists for a module assembly structure that can accommodate time-varying cell dimensional changes as well as provide a specified compression force to each cell.